Load-based vehicle operating control

ABSTRACT

A hybrid vehicle includes an engine and an electric machine, both capable of propelling the vehicle. The electric machine is electrically connected to a high voltage traction battery. The state of charge of the battery can decrease if the battery is used to power the electric machine, and can increase if the electric machine supplies power to the battery via regenerative braking. Constraints are placed on the vehicle such that the battery operates within a preferred operating window, defined between minimum and maximum state of charge thresholds. At least one controller is programmed to alter the preferred operating window of the battery in response to various vehicular activities, such as when the vehicle is towing another object, or when the vehicle weighs above a certain threshold due to contents within the vehicle, for example.

TECHNICAL FIELD

The present disclosure relates to controlling an engine and an electricmachine in a hybrid vehicle based upon vehicle load.

BACKGROUND

Hybrid electric vehicles (HEVs) include an internal combustion engineand a motor/generator to provide propulsion to the vehicle. One methodof increasing fuel economy in the HEV is to shut down the engine,leaving the motor/generator as the sole source of torque to propel thevehicle. This can occur during times of low overall torque demands, suchas when the vehicle is motionless, idling, creeping, or coasting, orhaving a relatively low overall acceleration demand. If, for example,the vehicle is on an incline, is subject to excessive weight in theinterior of the vehicle, or is towing another vehicle, the amount oftorque required to meet the acceleration demands of the operatorincreases. If the engine is disconnected while high acceleration demandsare made and while the vehicle is subject to these excessive loads, themotor/generator alone may be insufficient to meet the driver demand.

SUMMARY

According to one embodiment, a vehicle comprises an engine, an electricmachine connected to a battery, and at least one controller. In responseto a charge of the battery reaching a minimum state-of-charge threshold,the controller inhibits the electric machine from propelling the vehicleto prevent an undercharge of the battery. In response to a charge of thebattery reaching a maximum state-of-charge threshold, the controllerinhibits regenerative braking to prevent overcharge of the battery. Thebattery preferably operates within the confines of the minimum andmaximum state-of-charge thresholds. In response to a detection of avehicle load condition, the controller alters one or both of the minimumstate-of-charge threshold and the maximum state-of-charge thresholds.This alters the confines of the minimum and maximum thresholds, changingwhen the battery is inhibited from propelling the vehicle and/orreceiving electric energy via regenerative braking. In one embodiment,the vehicle load condition is a weight of the vehicle exceeding apredetermined weight threshold. In another embodiment, the vehicle loadcondition is when the vehicle is towing another object.

According to another embodiment, a method of controlling ahybrid-electric vehicle is provided. The method includes inhibiting atraction battery from powering an electric motor in response to astate-of-charge of the battery reaching a minimum state-of-chargethreshold. The method also includes altering the minimum state-of-chargethreshold in response to a load of the vehicle exceeding a predeterminedthreshold.

Another method of controlling a hybrid electric vehicle is providedaccording to another embodiment of the present disclosure. The methodincludes inhibiting an electric machine from generating electric powerbased upon a state-of-charge of a traction battery reaching a maximumstate-of-charge threshold. Furthermore, in response to a load of thevehicle exceeding a predetermined threshold, the maximum state-of-chargethreshold is altered.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a powertrain of a hybrid electricvehicle.

FIG. 2 is a flow chart of an algorithm for controlling the vehicle basedon the vehicle being laden or towing.

FIG. 3 is a flow chart of an algorithm for controlling the vehicle basedon the vehicle being subject to excessive loads.

FIG. 4 is a graph of an SOC operating range of a batter and a torquedistribution in a powertrain over time while the vehicle is subject toexcessive loads.

FIG. 5 is an example of an interactive information display within adashboard of the vehicle for commanding the vehicle to operate in a towmode.

DETAILED DESCRIPTION

Embodiments of the present disclosure are described herein. It is to beunderstood, however, that the disclosed embodiments are merely examplesand other embodiments can take various and alternative forms. Thefigures are not necessarily to scale; some features could be exaggeratedor minimized to show details of particular components. Therefore,specific structural and functional details disclosed herein are not tobe interpreted as limiting, but merely as a representative basis forteaching one skilled in the art to variously employ the embodiments. Asthose of ordinary skill in the art will understand, various featuresillustrated and described with reference to any one of the figures canbe combined with features illustrated in one or more other figures toproduce embodiments that are not explicitly illustrated or described.The combinations of features illustrated provide representativeembodiments for typical applications. Various combinations andmodifications of the features consistent with the teachings of thisdisclosure, however, could be desired for particular applications orimplementations.

Referring to FIG. 1, a schematic diagram of a hybrid electric vehicle(HEV) 10 is illustrated according to an embodiment of the presentdisclosure. FIG. 1 illustrates representative relationships among thecomponents. Physical placement and orientation of the components withinthe vehicle may vary. The HEV 10 includes a powertrain 12. Thepowertrain 12 includes an engine 14 that drives a transmission 16, whichmay be referred to as a modular hybrid transmission (MHT). As will bedescribed in further detail below, transmission 16 includes an electricmachine such as an electric motor/generator (M/G) 18, an associatedtraction battery 20, a torque converter 22, and a multiple step-ratioautomatic transmission, or gearbox 24.

The engine 14 and the M/G 18 are both drive sources for the HEV 10. Theengine 14 generally represents a power source that may include aninternal combustion engine such as a gasoline, diesel, or natural gaspowered engine, or a fuel cell. The engine 14 generates an engine powerand corresponding engine torque that is supplied to the M/G 18 when adisconnect clutch 26 between the engine 14 and the M/G 18 is at leastpartially engaged. The M/G 18 may be implemented by any one of aplurality of types of electric machines. For example, M/G 18 may be apermanent magnet synchronous motor. Power electronics 56 conditiondirect current (DC) power provided by the battery 20 to the requirementsof the M/G 18, as will be described below. For example, powerelectronics may provide three phase alternating current (AC) to the M/G18.

When the disconnect clutch 26 is at least partially engaged, power flowfrom the engine 14 to the M/G 18 or from the M/G 18 to the engine 14 ispossible. For example, the disconnect clutch 26 may be engaged and M/G18 may operate as a generator to convert rotational energy provided by acrankshaft 28 and M/G shaft 30 into electrical energy to be stored inthe battery 20. The disconnect clutch 26 can also be disengaged toisolate the engine 14 from the remainder of the powertrain 12 such thatthe M/G 18 can act as the sole drive source for the HEV 10. Shaft 30extends through the M/G 18. The M/G 18 is continuously drivablyconnected to the shaft 30, whereas the engine 14 is drivably connectedto the shaft 30 only when the disconnect clutch 26 is at least partiallyengaged.

The M/G 18 is connected to the torque converter 22 via shaft 30. Thetorque converter 22 is therefore connected to the engine 14 when thedisconnect clutch 26 is at least partially engaged. The torque converter22 includes an impeller fixed to M/G shaft 30 and a turbine fixed to atransmission input shaft 32. The torque converter 22 thus provides ahydraulic coupling between shaft 30 and transmission input shaft 32. Thetorque converter 22 transmits power from the impeller to the turbinewhen the impeller rotates faster than the turbine. The magnitude of theturbine torque and impeller torque generally depend upon the relativespeeds. When the ratio of impeller speed to turbine speed issufficiently high, the turbine torque is a multiple of the impellertorque. A torque converter bypass clutch 34 may also be provided that,when engaged, frictionally or mechanically couples the impeller and theturbine of the torque converter 22, permitting more efficient powertransfer. The torque converter bypass clutch 34 may be operated as alaunch clutch to provide smooth vehicle launch. Alternatively, or incombination, a launch clutch similar to disconnect clutch 26 may beprovided between the M/G 18 and gearbox 24 for applications that do notinclude a torque converter 22 or a torque converter bypass clutch 34. Insome applications, disconnect clutch 26 is generally referred to as anupstream clutch and launch clutch 34 (which may be a torque converterbypass clutch) is generally referred to as a downstream clutch.

The gearbox 24 may include gear sets (not shown) that are selectivelyplaced in different gear ratios by selective engagement of frictionelements such as clutches and brakes (not shown) to establish thedesired multiple discrete or step drive ratios. The friction elementsare controllable through a shift schedule that connects and disconnectscertain elements of the gear sets to control the ratio between atransmission output shaft 36 and the transmission input shaft 32. Thegearbox 24 is automatically shifted from one ratio to another based onvarious vehicle and ambient operating conditions by an associatedcontroller, such as a powertrain control unit (PCU) 50. The gearbox 24then provides powertrain output torque to output shaft 36.

It should be understood that the hydraulically controlled gearbox 24used with a torque converter 22 is but one example of a gearbox ortransmission arrangement; any multiple ratio gearbox that accepts inputtorque(s) from an engine and/or a motor and then provides torque to anoutput shaft at the different ratios is acceptable for use withembodiments of the present disclosure. For example, gearbox 24 may beimplemented by an automated mechanical (or manual) transmission (AMT)that includes one or more servo motors to translate/rotate shift forksalong a shift rail to select a desired gear ratio. As generallyunderstood by those of ordinary skill in the art, an AMT may be used inapplications with higher torque requirements, for example.

As shown in the representative embodiment of FIG. 1, the output shaft 36is connected to a differential 40. The differential 40 drives a pair ofwheels 42 via respective axles 44 connected to the differential 40. Thedifferential transmits approximately equal torque to each wheel 42 whilepermitting slight speed differences such as when the vehicle turns acorner. Different types of differentials or similar devices may be usedto distribute torque from the powertrain to one or more wheels. In someapplications, torque distribution may vary depending on the particularoperating mode or condition, for example.

The powertrain 12 further includes an associated powertrain control unit(PCU) 50. While illustrated as one controller, the PCU 50 may be part ofa larger control system and may be controlled by various othercontrollers throughout the vehicle 10, such as a vehicle systemcontroller (VSC). It should therefore be understood that the powertraincontrol unit 50 and one or more other controllers can collectively bereferred to as a “controller” that controls various actuators inresponse to signals from various sensors to control functions such asstarting/stopping engine 14, operating M/G 18 to provide wheel torque orcharge battery 20, select or schedule transmission shifts, etc.Controller 50 may include a microprocessor or central processing unit(CPU) in communication with various types of computer readable storagedevices or media. Computer readable storage devices or media may includevolatile and nonvolatile storage in read-only memory (ROM),random-access memory (RAM), and keep-alive memory (KAM), for example.KAM is a persistent or non-volatile memory that may be used to storevarious operating variables while the CPU is powered down.Computer-readable storage devices or media may be implemented using anyof a number of known memory devices such as PROMs (programmableread-only memory), EPROMs (electrically PROM), EEPROMs (electricallyerasable PROM), flash memory, or any other electric, magnetic, optical,or combination memory devices capable of storing data, some of whichrepresent executable instructions, used by the controller in controllingthe engine or vehicle.

The controller communicates with various engine/vehicle sensors andactuators via an input/output (I/O) interface that may be implemented asa single integrated interface that provides various raw data or signalconditioning, processing, and/or conversion, short-circuit protection,and the like. Alternatively, one or more dedicated hardware or firmwarechips may be used to condition and process particular signals beforebeing supplied to the CPU. As generally illustrated in therepresentative embodiment of FIG. 1, PCU 50 may communicate signals toand/or from engine 14, disconnect clutch 26, M/G 18, launch clutch 34,transmission gearbox 24, and power electronics 56. Although notexplicitly illustrated, those of ordinary skill in the art willrecognize various functions or components that may be controlled by PCU50 within each of the subsystems identified above. Representativeexamples of parameters, systems, and/or components that may be directlyor indirectly actuated using control logic executed by the controllerinclude fuel injection timing, rate, and duration, throttle valveposition, spark plug ignition timing (for spark-ignition engines),intake/exhaust valve timing and duration, front-end accessory drive(FEAD) components such as an alternator, air conditioning compressor,battery charging, regenerative braking, M/G operation, clutch pressuresfor disconnect clutch 26, launch clutch 34, and transmission gearbox 24,and the like. Sensors communicating input through the I/O interface maybe used to indicate turbocharger boost pressure, crankshaft position(PIP), engine rotational speed (RPM), wheel speeds (WS1, WS2), vehiclespeed (VSS), coolant temperature (ECT), intake manifold pressure (MAP),accelerator pedal position (PPS), ignition switch position (IGN),throttle valve position (TP), air temperature (TMP), exhaust gas oxygen(EGO) or other exhaust gas component concentration or presence, intakeair flow (MAF), transmission gear, ratio, or mode, transmission oiltemperature (TOT), transmission turbine speed (TS), torque converterbypass clutch 34 status (TCC), deceleration or shift mode (MDE), forexample.

Control logic or functions performed by PCU 50 may be represented byflow charts or similar diagrams in one or more figures. These figuresprovide representative control strategies and/or logic that may beimplemented using one or more processing strategies such asevent-driven, interrupt-driven, multi-tasking, multi-threading, and thelike. As such, various steps or functions illustrated may be performedin the sequence illustrated, in parallel, or in some cases omitted.Although not always explicitly illustrated, one of ordinary skill in theart will recognize that one or more of the illustrated steps orfunctions may be repeatedly performed depending upon the particularprocessing strategy being used. Similarly, the order of processing isnot necessarily required to achieve the features and advantagesdescribed herein, but is provided for ease of illustration anddescription. The control logic may be implemented primarily in softwareexecuted by a microprocessor-based vehicle, engine, and/or powertraincontroller, such as PCU 50. Of course, the control logic may beimplemented in software, hardware, or a combination of software andhardware in one or more controllers depending upon the particularapplication. When implemented in software, the control logic may beprovided in one or more computer-readable storage devices or mediahaving stored data representing code or instructions executed by acomputer to control the vehicle or its subsystems. The computer-readablestorage devices or media may include one or more of a number of knownphysical devices which utilize electric, magnetic, and/or opticalstorage to keep executable instructions and associated calibrationinformation, operating variables, and the like.

An accelerator pedal 52 and a brake pedal 53 are used by the driver ofthe vehicle to provide a demanded torque, power, braking, or drivecommand to propel the vehicle. In general, depressing and releasing theaccelerator pedal 52 generates an accelerator pedal position signal thatmay be interpreted by the controller 50 as a demand for increased poweror decreased power, respectively. Based at least upon input from theaccelerator pedal, the controller 50 commands torque from the engine 14and/or the M/G 18. Depression of the brake pedal 53 activatesregenerative braking and/or friction braking to decelerate the vehicle.

The controller 50 also controls the timing of gear shifts within thegearbox 24, as well as engagement or disengagement of the disconnectclutch 26 and the torque converter bypass clutch 34. Like the disconnectclutch 26, the torque converter bypass clutch 34 can be modulated acrossa range between the engaged and disengaged positions. This produces avariable slip in the torque converter 22 in addition to the variableslip produced by the hydrodynamic coupling between the impeller and theturbine. Alternatively, the torque converter bypass clutch 34 may beoperated as locked or open without using a modulated operating modedepending on the particular application.

To drive the vehicle with the engine 14, the disconnect clutch 26 is atleast partially engaged to transfer at least a portion of the enginetorque through the disconnect clutch 26 to the M/G 18, and then from theM/G 18 through the torque converter 22 and gearbox 24. The M/G 18 mayassist the engine 14 by providing additional power to turn the shaft 30.This operation mode may be referred to as a “hybrid mode” or an“electric assist mode.”

To drive the vehicle with the M/G 18 as the sole power source, the powerflow remains the same except the disconnect clutch 26 isolates theengine 14 from the remainder of the powertrain 12. Combustion in theengine 14 may be disabled or otherwise OFF during this time to conservefuel. The traction battery 20 transmits stored electrical energy throughwiring 54 to power electronics 56 that may include an inverter, forexample. The power electronics 56 convert DC voltage from the battery 20into AC voltage to be used by the M/G 18. The PCU 50 commands the powerelectronics 56 to convert voltage from the battery 20 to an AC voltageprovided to the M/G 18 to provide positive or negative torque to theshaft 30. This operation mode may be referred to as an “electric only”operation mode.

In any mode of operation, the M/G 18 may act as a motor and provide adriving force for the powertrain 12. Alternatively, the M/G 18 may actas a generator and convert kinetic energy from the powertrain 12 intoelectric energy to be stored in the battery 20. The M/G 18 may act as agenerator while the engine 14 is providing propulsion power for thevehicle 10, for example. The M/G 18 may additionally act as a generatorduring times of regenerative braking in which rotational energy fromspinning wheels 42 is transferred back through the gearbox 24 and isconverted into electrical energy for storage in the battery 20.

It should be understood that the schematic illustrated in FIG. 1 ismerely exemplary and is not intended to be limited. Other configurationsare contemplated that utilize selective engagement of both an engine anda motor to transmit through the transmission. For example, the M/G 18may be offset from the crankshaft 28, an additional motor may beprovided to start the engine 14, and/or the M/G 18 may be providedbetween the torque converter 22 and the gearbox 24. Other configurationsare contemplated without deviating from the scope of the presentdisclosure.

In typical hybrid vehicles, such as the vehicle 10 shown in FIG. 1,control strategies are provided in the controller 50 to maintain thestate-of-charge (SOC) of the battery 20 within a preferred predeterminedoperating window. For example, it may be desirable to maintain the SOCof the battery 20 during operation within a window of 40% and 60% offull charge. Other such predetermined operating windows are availabledepending on the needs of each individual vehicle.

If the SOC approaches the lower end of the preferred operating window,then the controller may command the disconnect clutch 26 to engage andthe engine 14 to start (if the engine is not already active). Enginepower can then be used to both propel the vehicle and charge the battery20 via the M/G 18 and power electronics 56. The controller can ceasecommands of electric propulsion by the M/G 18 in order to conserveelectric energy in the battery 20.

If the SOC approaches the higher end of the preferred operating window,the controller may command an electric-only operating mode bydisconnecting the engine 14 from the M/G 18 (if the engine is notalready disconnected). The controller may also use the M/G 18 to addtorque to the powertrain while reducing engine torque. Use of thebattery 20 to propel the vehicle 10 while reducing or eliminating torqueoutput by the engine 14 will work to drain excess charge from thebattery 20 to maintain the SOC within the preferred operating window.The controller may also cease any regenerative braking to inhibit theSOC from increasing past the higher end of the preferred operatingwindow.

During times of excessive vehicle load or when the vehicle is towinganother vehicle, it is particularly desirable to make high amounts oftorque immediately available for propelling the vehicle. Excessive loadsplaced upon the vehicle may tend to delay the response of accelerationdemands, especially when the vehicle is accelerating from rest.According to various embodiments of the present disclosure, a system isprovided to alter the preferred SOC operating window based upon thevehicle load and/or when the vehicle is towing an object. Furthermore,the system can activate the engine based upon the release of a brakepedal when the vehicle is at rest and subject to these excess loads.

FIG. 2 illustrates an exemplary flowchart of an algorithm 200 forcontrolling the operation of the powertrain based upon the excessiveloads. At 202, the controller determines whether the vehicle is subjectto excessive loads (e.g., laden, towing, inclined, etc.) This can bedetermined in many various fashions. For example, the excessive load canbe indicated by a force acting upon the axles of the vehicle over athreshold force to indicate the weight of the vehicle and its contents.If this weight is above a threshold, this indicates the vehicle issubject to the “excessive loads.” Other areas of the vehicle can also bemonitored to indicate the weight of the vehicle.

As another example, the excessive load can be indicated by the vehicletowing an object, such as another vehicle, a trailer, etc. A controllerin the vehicle can determine that the vehicle is towing another objectin many fashions, such as receiving a signal from a tension sensor at atow hitch on the vehicle, receiving a signal indicating an operator'scommand that the vehicle operate in a “tow mode (additional discussionprovided in FIG. 5 below), detecting an object immediately behind thevehicle by utilizing rearward-facing cameras at the back of the vehicle,receiving a signal from an electronic sensor indicating a connectionbetween tail lights of the vehicle and the towed vehicle. Other systemsexist and are contemplated as indicating that the vehicle is towing. Ineach of these situations in which the vehicle is towing another object,or in which the vehicle is subject to a heavy weight, the vehicle can besaid to be subject to “excessive loads” as the term is used in thepresent disclosure.

If it is indeed determined that the vehicle is subject to excessiveloads at 202, the controller can enable an engine-start based upon therelease of the brake pedal 53 at 204. For example, the controller maydetermine that the vehicle is stopped and the brake pedal is depressedto maintain the vehicle at zero velocity. While the vehicle is stopped,the controller continuously monitors the position of the brake pedal. Ifthe operator of the vehicle releases the brake pedal, it can be inferredthat an accelerator pedal depression (and thus a desire to acceleratethe vehicle) is imminent. Therefore, based upon the tip-out or releaseof the brake pedal, the controller engages the disconnect clutch 26 andstarts the engine 14 (if not already on). The starting of the engineincreases the amount of immediately-available torque in the powertrain.This enables the vehicle that is subjected to the excessive loads toadequately accelerate and fulfill driver demand more so than if thevehicle were to accelerate from rest while in an electric-only operatingmode.

If it is determined that the vehicle is subject to excessive loads at202, the controller can also expand the preferred operating window ofthe battery 20 at 206. Both the lower end (e.g., 40% of total charge)and the higher end (e.g., 60% of total charge) are altered such that awider preferred operating window is provided. When the lower end of theoperating window is decreased (e.g., to 30% of total charge), thebattery 20 will be enabled to be commanded by the controller to provideelectric assist during acceleration at greater amounts or longer timesso as to provide for a better electric “boost” while the vehicle issubject to the excessive loads. When the upper end of the operatingwindow is increased (e.g., to 70% of total charge), the M/G 18 will beenabled to be commanded by the controller to generate additional power(via regenerative braking, for example) and store additional power inthe battery 20. This accounts for the longer braking distances and/orthe increased amount of braking torque necessary to slow the vehiclewhen the vehicle is subjected to the excessive loads.

If, however, it is determined at 202 that the vehicle is not subject tothe excessive loads, the vehicle operates normally. At 208, thecontroller may not specifically enable an engine start based on therelease of the brake pedal. The controller also may not specificallyalter the SOC preferred operating window at 210. In other words, at 208and 210 the system returns such that the controller continually checksfor excessive loads at 202.

FIG. 3 illustrates an additional flowchart of an algorithm to controlthe powertrain during times that the vehicle is subject to excessiveloads. At 302, the controller determines whether the vehicle is towing,based upon the exemplary methods described above. If the vehicle is nottowing, the controller determines at 304 whether the weight of thevehicle is above a threshold, indicating the vehicle is subject to anexcessive load. If neither of these determinations produce a YES, themethod returns and continuously checks for either a towing or anexcessive load at 302 and 304.

If the vehicle is either towing or subjected to a weight exceeding apredetermined threshold, the vehicle is determined to be subject toexcessive load and the method proceeds to 306. At 306, the controllerdetermines what shift gear (e.g., Park, Reverse, Neural, Drive) thevehicle is in. A determination that the vehicle is in Drive indicatesthat the driver is traveling and intends to begin (or continue) movingforward. Therefore, a future acceleration event may be inferred from thevehicle being in Drive.

If the controller determines the vehicle is in Drive, then at 308, thecontroller commands the preferred operating window of the SOC of thebattery to expand. For example, the controller will alter the preferredoperating window from a range of 35%-80% to a range of 25%-85%. Aspreviously discussed, this enables the battery to provide for longer ormore powerful electric assist when commanded to do so, as well asutilize regenerative braking during a braking event for a longerdistance. The widened SOC preferred operating window therefore providesadvantages to the vehicle when additional acceleration and brakingdemands are needed due to the excessive loads.

With the SOC preferred operating window expanded, the controller alsocontinuously monitors the status of the brake pedal. If the vehiclecomes to a rest or reduces to a speed below a threshold, and theoperator subsequently releases the brake pedal as determined at 310,then the method proceeds to 312. At 312, the controller determines thestatus of the disconnect clutch 26 and/or the status of the engine 14.If the disconnect clutch 26 is disengaged, the vehicle is operating inan electric-only mode of operation. In order to increase the amount ofimmediately available torque from the engine, the controller engages thedisconnect clutch 26 and commands ignition in the engine 14 at 314. Withthe engine engaged at the time the brake pedal is released, the engineis available to provide torque immediately in response to a depressionof the accelerator pedal 52. This enables the vehicle to more properlymeet driver demand when the vehicle is subjected to the excessive loads.

FIG. 4 illustrates an example of certain characteristics of a vehiclethat employs the control system of the present disclosure. In thisexample, the vehicle begins at rest while being subjected to theexcessive loads, and the operator of the vehicle provides a heavy tip-inof the accelerator pedal, indicating the need for a high amount ofimmediate torque to quickly accelerate the vehicle.

At time T₀, the vehicle is at rest. The battery has been charged suchthat the SOC is at the upper bound of its expanded preferred operatingwindow. In other words, the controller in the vehicle has determinedthat the vehicle is subject to the excessive load, and has thereforeexpanded the SOC operating window so that the battery is fully chargedat the maximum of the expanded SOC window.

At time T₁, the operator of the vehicle has released the brake pedal inanticipation of accelerating the vehicle. Based on the release of thebrake pedal, the controller also commands the engine to turn on byengaging the disconnect clutch 26 and commanding ignition in the engine14. Due to the starting of the engine, the max torque (Tq_max_eng) risesto its maximum point from T₁ to T₂.

At time T₂, the engine has fully started and is running. Theacceleration demand remains zero. Due to the additional available torqueprovided by the fully-started engine, the maximum torque at thepowertrain (Tq_max_pt) is equal to the sum of the maximum torque of theengine (Tq_max_eng) and the maximum torque of the M/G (Tq_max_mot).

At time T₃, the operator of the vehicle depresses the accelerator pedal.With the full powertrain torque (Tq_max_pt) already at its maximum, thedriver's demand is met. The vehicle can therefore properly provide thetorque and acceleration according to the driver demand dictated by thedepression of the accelerator pedal. From T₃ to T₄, the driver demandexceeds the maximum torque in the engine (Tq_max_eng). The M/G thereforesupplements the torque of the engine to meet the driver demand. The SOCof the battery depletes during the use of the M/G. However, since theSOC range has been expanded, use of the M/G takes longer to deplete thebattery to the lower bounds of its operating window. This provides abetter opportunity for the M/G to remain an active torque provider for alonger time and therefore better meet driver demand during the entireacceleration event.

At time T4, the SOC has depleted to its lower boundary of the SOCoperating window. Available battery current is therefore zero. Becauseof this, the powertrain torque (Tq_max_pt) reduces to a level equal withthat of the engine alone (Tq_max_eng), and the engine remains the soletorque provider during the remainder of the acceleration event.

As previously discussed, there are multiple ways in which the controllerin the vehicle can determine if the vehicle is towing another object.FIG. 5 illustrates one such example, in which an interactive informationdisplay 500 is disposed within a dashboard 502 of the vehicle, such asin an instrument panel or a center console area. The information display500 may be part of another display system, such as a navigation displaysystem, or may be part of a dedicated information display system. Theinformation display 500 may be a liquid crystal display (LCD), a plasmadisplay, an organic light emitting display (OLED), or any other suitabledisplay. The information display 500 may include a touch screen forreceiving driver input associated with selected areas of the screen ofthe information display 500. One or more buttons 504 may be included onthe information display 500.

The operator of the vehicle may navigate to a screen in which a tow modeselection is available. At this screen, the operator can manually directthe controller of the vehicle to enter a “tow mode” such that thevehicle automatically operates as if it is subject to the excessiveloads. If the operator utilizes the information display 500 to enter the“tow mode,” the controller will respond with a YES for steps 302 and/or304 of the previously described method such that the SOC operatingwindow can be widened and the engine can be enabled to engage upon therelease of the brake pedal.

The processes, methods, or algorithms disclosed herein can bedeliverable to/implemented by a processing device, controller, orcomputer, which can include any existing programmable electronic controlunit or dedicated electronic control unit. Similarly, the processes,methods, or algorithms can be stored as data and instructions executableby a controller or computer in many forms including, but not limited to,information permanently stored on non-writable storage media such as ROMdevices and information alterably stored on writeable storage media suchas floppy disks, magnetic tapes, CDs, RAM devices, and other magneticand optical media. The processes, methods, or algorithms can also beimplemented in a software executable object. Alternatively, theprocesses, methods, or algorithms can be embodied in whole or in partusing suitable hardware components, such as Application SpecificIntegrated Circuits (ASICs), Field-Programmable Gate Arrays (FPGAs),state machines, controllers or other hardware components or devices, ora combination of hardware, software and firmware components.

While exemplary embodiments are described above, it is not intended thatthese embodiments describe all possible forms encompassed by the claims.The words used in the specification are words of description rather thanlimitation, and it is understood that various changes can be madewithout departing from the spirit and scope of the disclosure. Aspreviously described, the features of various embodiments can becombined to form further embodiments of the invention that may not beexplicitly described or illustrated. While various embodiments couldhave been described as providing advantages or being preferred overother embodiments or prior art implementations with respect to one ormore desired characteristics, those of ordinary skill in the artrecognize that one or more features or characteristics can becompromised to achieve desired overall system attributes, which dependon the specific application and implementation. These attributes caninclude, but are not limited to cost, strength, durability, life cyclecost, marketability, appearance, packaging, size, serviceability,weight, manufacturability, ease of assembly, etc. As such, embodimentsdescribed as less desirable than other embodiments or prior artimplementations with respect to one or more characteristics are notoutside the scope of the disclosure and can be desirable for particularapplications.

What is claimed is:
 1. A vehicle comprising: an engine; an electricmachine; a battery connected to the electric machine; and at least onecontroller programmed to (i) inhibit the electric machine frompropelling the vehicle in response to a charge of the battery reaching aminimum state-of-charge threshold, (ii) inhibit regenerative braking inresponse to the charge reaching a maximum state-of-charge threshold, and(iii) alter at least one of the thresholds in response to detecting avehicle load condition.
 2. The vehicle of claim 1, wherein the at leastone controller is further programmed to decrease the minimumstate-of-charge threshold in response to detecting the vehicle loadcondition to increase an amount of available electric power.
 3. Thevehicle of claim 1, wherein the at least one controller is furtherprogrammed to increase the maximum state-of-charge threshold in responseto detecting the vehicle load condition to enable an increased amount ofelectric energy generation.
 4. The vehicle of claim 1, wherein thevehicle load condition is defined by a weight of the vehicle exceeding apredefined value.
 5. The vehicle of claim 1, wherein the vehicle loadcondition is defined by a state in which an object is being towed. 6.The vehicle of claim 1, further comprising a brake pedal and a clutchconfigured to selectively couple the engine to the electric machine,wherein the at least one controller is further programmed to start theengine in response to (i) a release of the brake pedal while the vehicleis stopped and in drive and (ii) detecting the vehicle load condition toincrease available engine torque prior to vehicle launch.
 7. A method ofcontrolling a hybrid-electric vehicle comprising: inhibiting a tractionbattery from powering an electric motor in response to a state-of-chargeof the battery reaching a minimum state-of-charge threshold; and inresponse to a load of the vehicle exceeding a predetermined threshold,altering the minimum state-of-charge threshold.
 8. The method of claim7, wherein the altering includes decreasing the minimum state-of-chargethreshold to increase an amount of available electric power.
 9. Themethod of claim 7, further comprising receiving a signal indicating thatan object is being towed by the vehicle, wherein the step of altering isfurther in response to the signal.
 10. The method of claim 7, whereinthe load of the vehicle is defined by a weight of the vehicle.
 11. Themethod of claim 7, further comprising starting an engine in response to(i) a release of a brake pedal while the vehicle is stopped and indrive, and (ii) the load of the vehicle exceeding the predeterminedthreshold.
 12. A method of controlling a hybrid electric vehiclecomprising: inhibiting an electric machine from generating electricpower based upon a state-of-charge of a traction battery reaching amaximum state-of-charge threshold; and in response to a load of thevehicle exceeding a predetermined threshold, altering the maximumstate-of-charge threshold.
 13. The method of claim 12, wherein thealtering includes increasing the maximum state-of-charge threshold toenable an increased amount of electric energy generation.
 14. The methodof claim 12, further comprising receiving a signal indicating that of anobject is being towed by the vehicle, wherein the step of altering isfurther in response to the signal.
 15. The method of claim 12, whereinthe load of the vehicle is defined by a weight of the vehicle.
 16. Themethod of claim 12, further comprising starting an engine in response toa (i) a release of a brake pedal while the vehicle is stopped and indrive, and (ii) the load of the vehicle exceeding the predeterminedthreshold.